The long-term objective of this project is to understand the role and unique signaling mechanism of the Rho family small GTPase Cdc42 in mammalian cells of various lineages. Current biochemical model depicts that upon ligand binding to cell surface receptors or in response to cellular stress, Cdc42 is activated to recognize specific downstream effectors to regulate multiple cell functions including actin reorganization, adhesion, migration, vesicle trafficking and cell cycle progression. To date, however, most studies of Cdc42 function in mammalian cells have been carried out by overexpressing dominant negative or constitutively active Cdc42 mutants in clonal cell lines;the physiologic role and signaling mechanism of Cdc42 in most primary cell lineages have yet to be examined genetically. We have generated the Cdc42 conditional knockout mice by a loxP/Cre recombination strategy to allow targeted deletion of Cdc42 in adult or embryonic tissues including various hematopoietic organs, and have produced a Cdc42 gain of activity mouse model by gene targeting of a Cdc42 negative regulator, Cdc42GAP, that causes a global upregulation of Cdc42 activity in most cell types. Based on our preliminary studies of cells derived from these Cdc42 loss or gain of activity mice, we hypothesize that Cdc42 regulates cell growth, migration and other cell-type specific functions in a cell- and stimulus-specific manner. To define the Cdc42 signaling mechanisms in mammalian primary cells that may not be revealed by conventional means, in this proposal we will utilize the Cdc42 loss and gain of activity mouse models (1) to study the role and signaling mechanism of Cdc42 in the hematopoietic stem/progenitor cell (HSC) lineage;(2) to determine the involvement of Cdc42 in regulating the unique functions of the HSC- differentiated mast cells, and (3) to examine the structure-function relationship of Cdc42 in the HSC and mast cells by a reconstitution approach. Human disease relevance: The results may implicate Cdc42 in hematopoietic stem/progenitor and mast cell regulation and associate Cdc42 mulfunction with human hematopoietic stem/progenitor and mast cell disorders such as leukemia and inflammation.